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The use of advanced electron microscopy techniques to characterize both the bulk and near-atomic level microstructural evolution of catalyst materials during different dynamometer/vehicle aging cycles is an integral part of understanding catalyst deactivation. The study described here was undertaken to evaluate thermally-induced microstructural changes which caused the progressive loss of catalyst performance in a three-way automotive catalyst. Several different catalyst processing variables, for example changing the washcoat ceria content, were also evaluated as a function of aging cycle and thermal history. A number of thermally-induced microstructural changes were identified using high resolution electron microscopy techniques that contributed to the deactivation of the catalyst, including sintering of all washcoat constituents, γ-alumina transforming to α-, β-, and δ-alumina, precious metal redistribution, and constituent encapsulation.

A study was performed to assess the effects of the catalyst volume and the ceria content in the washcoat on the aged emission performance of underfloor catalytic converters containing platinum and rhodium. Catalyst volumes of 1.4 L and 2.8 L were evaluated, while the ceria level was varied from 0 to 60% of the weight of the washcoat. The concentration of noble metals (g/L) was the same for both catalyst volumes, so the larger volume also contained more noble metal. Catalyst performance was evaluated on an air/fuel ratio sweep test, at steady-state conditions on an engine, and on the FTP test. In light of the new catalyst monitoring requirements for OBD II, each catalyst was also evaluated at steady-state conditions using a dual oxygen sensor technique in order to produce an O2 sensor index. The evaluations were performed at several intermediate stages as the catalysts were aged on engines using high temperature durability schedules intended to simulate high mileage conditions.

With the world-wide growth of the automotive emissions controls market, concerns about the future cost and availability of catalytic metals, particularly rhodium, have also grown. These factors have led to an increased interest in catalyst formulations which might allow reduced Rh usage or the complete removal of Rh from the catalyst without compromising the performance of emissions control systems. We have tested a set of catalysts to examine Ru, Ir, and Pd as alternatives to Rh, either alone or in combination with Pt. For the nine catalysts of Pt, Rh, Pd, Ru, Ir, Pt/Rh, Pt/Pd, Pt/Ru, and Pt/Ir studied, the loading of all constituent metals on 85 cu. in. monoliths in single or 1:1 dual component catalysts was 0.038 oz t, except for Rh which had a 0.0038 oz t loading. Most of the monoliths were evaluated after 0, 6, and 75 hours on a rapid aging test schedule using sweep, light-off, dynamometer and/or vehicle tests using the Federal Test Procedure (FTP).